A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g. a silicon wafer). The pattern on the patterning device may correspond to an individual layer of an IC which is projected onto a target portion (e.g. comprising part of, one or several dies) on the substrate (e.g. a silicon wafer). In general, a single substrate will contain a network of adjacent target portions that are successively exposed.
A pattern which is projected by the lithographic apparatus is aligned with a pattern already present on a target portion of the substrate. In order to achieve this the lithographic apparatus is provided with an alignment system. The alignment system measures the positions of alignment marks provided on the substrate. These measurements are used to position the substrate during projection of the pattern onto the substrate.
It may be desirable to provide, for example, an alignment system which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.